1. Field of the Invention
The present invention relates generally to electrical circuits and, more particularly, to voltage-controlled oscillators.
2. Related Art
Frequency synthesizers, such as a multi-rate fractional-N frequency synthesizer, are well known. In a typical frequency synthesizer (e.g., a phase-locked loop), a frequency adjustment is required when the maximum or minimum frequency of the voltage-controlled oscillator (VCO) is not sufficient to synthesize the desired frequency. A common technique is to adjust the center frequency of the VCO by laser trimming certain circuit elements, such as a capacitor or a resistor, that control the VCO frequency to adjust the VCO center frequency to within its frequency-capture range.
One drawback of physical trimming is that the range of frequencies is permanently limited. For example, in a fractional-N synthesizer, which is capable of synthesizing hundreds of frequencies (e.g., each selected by a set of programmed bits), physical trimming permanently constrains the range of frequencies that can be synthesized to less than what could possibly be generated by the synthesizer. Furthermore, physical trimming incurs additional cost and time because the VCO frequency for each synthesizer must be measured and trimmed as necessary.
A VCO frequency range detector is disclosed herein. The VCO frequency range detector detects when the VCO frequency is out of range and indicates the required direction of the frequency adjustment. The VCO frequency range detector may be used, for example, as part of a VCO frequency-trimming block in a multi-rate fractional-N frequency synthesizer. Physical trimming is thus eliminated by introducing on-chip trimming circuitry that adjusts the VCO frequency when it is out of range by executing adjustments based on the signals received from the VCO frequency range detector.
In accordance with one embodiment of the present invention, a voltage-controlled oscillator frequency range detector includes a comparator circuit, which receives a voltage-controlled oscillator control voltage signal and provides at least one output signal based on a comparison of the voltage-controlled oscillator control voltage signal to a maximum or minimum voltage level, and a frequency detector circuit, which receives a reference clock signal and a voltage-controlled oscillator clock signal and provides an output signal indicating which is at a greater frequency. The voltage-controlled oscillator frequency range detector further includes a decision logic circuit coupled to the comparator circuit and to the frequency detector circuit. The decision logic circuit receives at least one output signal from the comparator circuit and the output signal from the frequency detector circuit and indicates whether a frequency adjustment to a voltage-controlled oscillator is required.
In accordance with another embodiment of the present invention, a method of detecting when a frequency of a voltage-controlled oscillator within a phase-locked loop is out of range includes receiving a voltage-controlled oscillator control voltage; comparing the voltage-controlled oscillator control voltage to a maximum voltage level and a minimum voltage level and providing a first result; receiving a reference clock signal and a voltage-controlled oscillator clock signal; comparing the reference clock signal to the voltage-controlled oscillator clock signal and providing a second result; and determining whether the frequency of the voltage-controlled oscillator is out of range based on the first result and the second result.
A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly.